Shear strength of the carbon-resin bond in carbon-fibre-reinforced epoxies

Abstract
The shear stiffness and strength of uniaxial specimens of carbon-fibre-reinforced epoxy resin have been measured by use of a torsional testing machine. Both type I (high stiffness) and type II (high strength) fibres were tested in the range 30-70% fibre by volume and the results compared with those for unreinforced resin. A quantitative study was made of the effect on these properties of the hypochlorite surface treatment designed to promote fibre-resin adhesion. The results have been analysed in terms of calculated stress concentration factors and the ideas of fracture mechanics. It is concluded that for type II fibres the surface treatment is fully effective in that resin failure is more likely than bond failure, at least for this particular resin. For type I fibres, however, a stronger treatment should be investigated, and possible improvements in the technique of manufacture of materials with high fibre content are indicated. A comparison has been made of torsional measurements with the results of short-beam interlaminar shear tests. The usefulness and limitations of the latter technique are assessed.